1. Field of the Invention
The present invention relates to a system for monitoring tire pressure, etc. for a vehicle such as a four-wheel automobile, etc.
2. Description of Related Art
In recent years, there is a tire pressure monitoring system (TPMS) as a system for monitoring a state of a tire in a vehicle such as a four-wheel automobile, etc. A system for monitoring temperature, distortion, a rotation number, acceleration, etc. in addition to pressure is also devised.
For example, TPMS has a vehicle body side controller able to transmit a wireless signal through an antenna for vehicle body side signal transmission respectively arranged near each tire of the vehicle. TPMS also has a sensor unit arranged in each tire of the vehicle and measuring the pressure of each tire and able to transmit its measuring result as a wireless signal. The above vehicle body side controller transmits a request signal (trigger signal) to the sensor unit of the corresponding tire from the antenna for vehicle body side signal transmission in a specific portion in predetermined timing. The above sensor unit receiving this request signal transmits an answer signal including the above measuring result to the above vehicle body side controller. The above vehicle body side controller receiving this answer signal reads the above measuring result. For example, when the measuring result shows an abnormal pressure, the vehicle body side controller executes control for outputting an alarm.
As the above TPMS, there is also a one-direction communication system for periodically transmitting the wireless signal including the measuring result from e.g., the sensor unit of the tire without arranging the antenna for vehicle body side signal transmission (an antenna constituting an initiator) at every tire. However, in this case, it is impossible for the vehicle body side controller to judge whether it is the measuring result of the tire in which position (e.g., a front wheel or a rear wheel and a left-hand side or a right-hand side in the case of a four-wheel vehicle) by changing the position of the tire in accordance with tire rotation. Therefore, for example, there is a disadvantage in that information as to whether the pressure abnormality is generated in the tire of which position cannot be notified to a vehicle user.
In contrast to this, as mentioned above, if it is set to a bidirectional communication system at every tire for arranging the antenna for vehicle body side signal transmission at every tire, and separately transmitting the request signal every tire and separately receiving the measuring result from the sensor unit of each tire as the answer signal with respect to this request signal, it is possible for the vehicle body side controller to judge whether it is the measuring result of the tire of which position.
When a cross talk in which the request signal transmitted to the sensor unit of a specific tire is also received in the sensor unit of another tire is generated in the above bidirectional communication system at every tire, it is impossible to finally discriminate the above tire position. Therefore, a communication range of the request signal from the antenna for vehicle body side signal transmission should be naturally limited to only a narrow range near the corresponding specific tire, and a transmitting output of the above request signal is also naturally limited.
However, on the other hand, the tire is rotated by running the vehicle, and the position of an antenna for signal reception in the sensor unit is also rotated and moved in accordance with the rotation of the tire. Therefore, the position and posture of the antenna for signal reception in the sensor unit of the tire side are always changed, and it becomes difficult to receive the above request signal in accordance with the rotating position of the tire. When the transmitting output of the above request signal is weak, there is a fear that the above request signal cannot be received and no communication can be performed.
Therefore, a construction for conforming each of directivities of the antenna for vehicle body side signal transmission (trigger signal transmitting means) and the antenna for signal reception of the tire side (trigger signal receiving means) to a rotating axis direction of the tire is conventionally proposed as can be seen in JP-A-2005-1498 (Patent Document 1) in the above bidirectional communication system every tire. When the directivities of the antenna for vehicle body side signal transmission and the antenna for signal reception of the tire side are conformed to the rotating axis direction of the tire, no direction of the directivity of the antenna for signal reception is changed even when the tire is rotated. Furthermore, the direction of the directivity of the antenna for signal reception is always conformed to the direction of the directivity of the antenna for vehicle body side signal transmission. Accordingly, the possibility that no communication can be performed as mentioned above is reduced.
However, when the above conventional construction (the construction for setting all the directivities of the respective antennas to the tire rotating axis direction) is set, it is impossible to sufficiently reduce the possibility that the cross talk in which the request signal is also received in the sensor unit of another tire is generated. Further, a problem exists in that this cross talk is easily generated on the contrary. The reasons for this are as follows. In the case of the above conventional construction, the directions of the directivities of the respective antennas (the antenna for vehicle body side signal transmission and the antenna for signal reception of the tire side) are conformed with respect to all the tires. Therefore, in the case of a vehicle in which the tires are mounted on both transversal sides of a four-wheel automobile, etc. so as to form a pair, it is easy to generate the cross talk in which the request signal transmitted to the sensor unit of the tire of e.g., a front wheel left-hand side is also received in the sensor unit of the tire of a front wheel right-hand side.